MOLECULAR-CLONING AND CHARACTERIZATION OF A NOVEL PROTEIN-KINASE WITHA CATALYTIC DOMAIN HOMOLOGOUS TO MITOGEN-ACTIVATED PROTEIN-KINASE KINASE KINASE

Mitogen-activated protein kinase (MAPK) signaling cascades include MAP K or extracellular signal-regulated kinase (ERK), MAPK kinase (MKK or MEK), and MAPK kinase kinase (MAPKKK or MEKK). MAPKK kinase/MEKK phosp horylates and activates its downstream protein kinase, MAPK kinase/MEK , which in turn activates MAPK. We report herein the isolation of a cD NA encoding a novel protein kinase designated MAP-KKK5 from a human ma crophage library. The nucleotide sequence predicts that MAP-KKK5 encod es an open reading frame of 1374 amino acids with all 11 kinase subdom ains. The putative catalytic domain of MAP-KKK5 shows significant sequ ence homology to the kinase domains of the MAPKKK/MEKK level protein k inases from mouse MEKK2 and -3, Drosophila melanogaster PK92B, Sacchar omyces cerevisiae STE11, and Schizosaccharomyces pombe BYR2. Northern blot analysis showed that MAPKKK5 transcript is abundantly expressed i n human heart and pancreas. When transiently expressed in COS and 293 cells, MAPKKK5 markedly activated c-Jun N-terminal kinase or stress-ac tivated protein kinase, but not MAPK/ERK. Furthermore, MAPKKK5 that wa s immunoprecipitated from transfected 293 cells was able to phosphoryl ate and activate MKK4 in vitro, suggesting that MAPKKK5 may be an upst ream activator of MKK4 in the c-Jun N terminal kinase pathway.